UNEXPECTED YELLOW-BLUE ELECTROCHROMISM OF ITO POWDERS AT MODEST POTENTIALS IN AQUEOUS-ELECTROLYTES

Tin-doped indium oxide (ITO) powders. both commercial and laboratory s amples. have been incorporated into diffuse-reflecting. interdigitated electrochromic displays using aqueous electrolytes. A surprising yell ow-blue electrochromism has been observed which has the features of op eration at low potential, minimal charge consumption (< 10 mC cm(-2)). reasonable switching speed (ca. 1s) and impressive lifetime (> 500 00 0 cycles). The observed electrochromic effect is attributed to a combi nation of changing the effective ITO band gap and populating/depopulat ing the conduction band. Smaller primary ITO crystallites were shown t o lead to an enhanced effect, as doss increasing tin dopant level to 2 5 mol%. Increases in dopant level beyond 25% gave no further improveme nt. Cyclic voltammetry showed no obvious Faradaic process associated w ith the color changes observed. In addition to obtaining diffuse refle ctance spectra, which show that contrast ratio for these displays actu ally changes direction at about 500 nm, we also measured CIE b-values , which give a quantitative indication of observed blue-yellow shift. These powders appear to be promising candidates for low cost, low info rmation content displays.